On her return voyage to the United States, she met English businessman William Henry Blatch, Jr., who went by the name of Harry. Blatch and Harriot Stanton were married in 1882, and lived outside of London for twenty years. They had two daughters, the second of whom died at age four. Their first daughter, Nora Stanton Blatch Barney, continued the family tradition as a suffragist, was the first American woman to earn a degree in civil engineering, and was briefly married to Lee De Forest. Harry Blatch died in 1915, after being accidentally electrocuted.

While in England, she performed a statistical study of rural English working women's conditions, for which she received her M.A. from Vassar. In the 1901 census Blatch is recorded as a visitor in Haslemere, Surrey in a house which formed part of the Haslemere Peasant Arts movement, a group which promoted the teaching of handicraft to rural women and girls. She also worked with English social reform groups, including the Women's Local Government Society, the Fabian Society, and the Women's Franchise League. In the Women's Franchise League, she developed organizing techniques that she would later use in America.

On returning to the United States in 1902, Blatch sought to reinvigorate the American women's suffrage movement, which had stagnated. She initially joined the leadership of the Women's Trade Union League. In 1907, she founded the Equality League of Self-Supporting Women (later renamed the Women's Political Union), to recruit working class women into the suffrage movement. The core membership of the league comprised 20,000 factory, laundry, and garment workers from the Lower East Side of New York City. Through this group, Blatch organized and led the 1910 New York suffrage parade. Blatch succeeded in mobilizing many working-class women, even as she continued to collaborate with prominent society women. She could organize militant street protests while still working expertly in backroom politics to neutralize the opposition of Tammany Hall politicians who feared the women would vote for prohibition.[2]

The Union achieved significant political strength, and actively lobbied for a New York state constitutional amendment to give women the vote, which was achieved in 1917 after Tammany Hall relaxed its opposition. In 1915, Blatch's Women's Political Union merged with Alice Paul and Lucy Burns' Congressional Union, which eventually became the National Woman's Party.

During World War I, Blatch devoted her time to the war effort, heading the Women's Land Army, which provided additional farm labor. She wrote Mobilizing Woman Power in 1918, about women's role in the war effort, urging women to "go to work". In 1920, she published A Woman's Point of View, where she took a pacifist position due to the destruction of the war.